As previously reported, myofibrillar ATPase activity and the duration of the isometric twitch in papillary muscles change with age and do not appear to be related. Maximum myofibrillar ATPase activity in detergent treated myofibrils decreased approximately twofold in rats between 2 and 6 months of age while the contraction duration of the isometric twitch increased progressively across the age span changing by approximately 30% in rats 24 months of age. It has been reported that animals treated with thyroxine increased the myosin ATPase activity and decreased the time to peak force, a component of twitch duration, and have suggested that the two parameters are related. Therefore, we treated young (2 mo), adult (8 mo), and senescent (24 mo) rats with thyroxine to produce a hyperthyroid state and were able to show that the contraction duration of the isometric twitch decreased without altering the maximum ATPase acitivity in any group. However, when myosin isoenzymes V1, V2, and V3 were examined, a redistribution of these isoenzymes occurred both with age and thyroxine treatment. Therefore, we conclude that age-associated prolongation of contraction duration is not fixed and can be reversed with thyroxine treatment. However, changes in myofibrillar ATPase activity are not associated with the changes in contraction duration produced by alterations of the thyroid state of the rat while changes were seen in the myosin isoenzyme patterns. This suggests that myosin ATPase and not myofibriallar ATPase may play an important role in contraction duration.